crate lib refactoring

- [x] replace `src/parser/mod.rs` with `src/parser.rs` and such.
- [x] Use `f.write_str()` (faster) instead of `write!()` (slower), if possible.
- [x] Don't export everything from crate root instead separate by `crate::{lexer, parser}`
- [x] move `struct Usage` and `struct See` into their own file.
- [x] ~reduce number of macros~ _not needed_
- [x] methods should have proper names and docs